(74d) Breakage of Alumina Aggregates Under Shear By CFD-DEM Simulations | AIChE

(74d) Breakage of Alumina Aggregates Under Shear By CFD-DEM Simulations


Goudeli, E. - Presenter, University of Melbourne
Zeng, L., The University of Melbourne
Franks, G., The University of Melbourne
Controlling dispersion, aggregation and breakage of fine particles is crucial in a range of industries, including wastewater treatment, mineral processing, and ceramic powder processing, as these mechanisms can affect particle characteristics and, in turn, the process efficiency and product quality (Kushimoto et al, 2020). For instance, the size of aggregates affects the physical properties of suspensions such as viscoelasticity and particle settling rate. Aggregation of fine valuable mineral particles can enable their collection by froth flotation. Even though almost all unit operations inevitably induce shear, their effect on aggregate characteristics is not fully understood due to difficulties in monitoring the temporal evolution of aggregate behavior at short time scales at the microscopic level. This challenge can be overcome by computer simulations of particle dynamics in suspensions under different shear environments.

Here, Computational Fluid Dynamics (CFD) are coupled with Discrete Element Method (DEM) to investigate the effect of shear on alumina aggregation accounting for particle-particle (surface, contact and collision forces) and particle-fluid interactions (hydrodynamic forces). The evolving aggregate size, size distribution, and aggregate structure are quantified during concurrent aggregation and breakage induced by shear (at shear rates, G, of 300 – 3000 s-1) in a Couette flow cell from perfect spherical particles all the way up to large aggregates, finding excellent agreement with experiments1. Snapshots of the aggregates formed by shear-induced aggregation undergoing simultaneous breakage are shown exemplarily for G = 1000 s-1 in Figure 1. Initially, spherical primary particles (blue spheres) collide with each other forming larger aggregates (e.g., white particles at 0.03 and 0.05 s). The particles grow further with time (red particles at 1 s) and, once steady state has been reached, the aggregation and breakage rate are both dominant resulting in internal aggregate restructuring without further particle growth. Aggregate breakage is quantified during steady state and a new equation is proposed for the breakage rate as a function of the shear rate and the aggregate size for the first time at low shear rates. Such easy-to-use breakage rate equations obtained from accurate CFD-DEM simulations can be readily used in population balance models.